Perovskite semiconductor thin film and method of making thereof

ABSTRACT

Perovskite semiconductor thin films and the method of making Perovskite semiconductor thin films are disclosed. Perovskite semiconductor thin films were deposited on inexpensive substrates such as glass and ceramics. CsSnI 3  films contained polycrystalline domains with typical size of 300 nm and larger. It is confirmed experimentally that CsSnI 3  compound in its black phase is a direct band-gap semiconductor, consistent with the calculated band structure from the first principles.

BACKGROUND OF THE INVENTION

Perovskite semicomductor thin films exhibit outstanding optical,electrical, and ferroelectric properties. This feature makes perovskitefilms ideally suited for a wide range of applications such as lightemitting and photovoltaic devices.

D. Scaife, P. Weller, and W. Fisher, reported an early study on thestructural information of CsSnI₃ compound in form of powders, J. SolidState Chem. 9, 308 (1974).

P. Mauersberger and F. Huber, synthesized a yellow, needle-like CsSnI₃microcrystal, and studied its crystal structure, Acta Cryst. B 36, 683(1980).

K. Yamada, S. Funabiki, H. Horimoto, T. Matsui, T. Okuda, and S. Ichiba,reported the polymorph nature of CsSnI₃ compound, Chem. Lett. (TheChemical Society of Japan) 20, 801 (1991).

The black polymorph of CsSnI₃ could be obtained through a phasetransition from the yellow polymorph CsSnI₃ by increasing itstemperature above 425 K. It was further demonstrated by differentialthermal analysis and X-ray diffraction that during the cooling of theblack CsSnI₃ from 450 K, its ideal cubic perovskite structure (B-α)deformed to a tetragonal structure (B-β) at 426 K, and became anorthorhombic structure (B-γ) below 351 K. Experimental studies ofelectrical and optical properties of this compound have been hindered bylack of high quality CsSnI₃ samples either in bulk or thin film format.

Aiming at the unique properties of hybrid organic-inorganic perovskitesbased on tin halides, I. Borriello, G. Gantel, and D. Ninno, recentlycalculated band structures of B-α, B-β, and B-γ from the firstprinciples using the crystal structures published by Yamada et al.,Phys. Rev. B 77, 235214 (2008). It was concluded that all threestructures had direct band-gap (E_(g)) at Z, R, and Γ points for B-α,B-β, and B-γ, respectively, with E_(g) (B-α)<E_(g) (B-β)<E_(g) (B-γ).

A need still exists in the industry for developing perovskitesemiconductor thin films, especially with large domain size. Thesuccessful implementation of these materials for various applicationrequires a detailed understanding of both their processing and materialsproperties.

BRIEF SUMMARY OF THE INVENTION

This invention is directed to large domain size high quality perovskitesemiconductor thin films and effective and inexpensive methods tosynthesize the films on large-area substrates such as glass, ceramicsand silicon.

One embodiment of this invention is directed to a polycrystallineperovskite semiconductor thin film comprising

CsM₁(M₂)₃;

wherein M₁ is selected from the group consisting of Sn, Pb andcombination thereof; and M₂ is selected from the group consisting of I,Cl, Br and combinations thereof.

Example for the above thin film are CsSnI₃: wherein M₁ is Sn; M₂ is I;and CsSnCl₃: wherein M₁ is Sn; M₂ is Cl₃; and CsSnBr₃: wherein M₁ is Sn;M₂ is Br.

Another embodiment of this invention is directed to the method forproducing a polycrystalline perovskite semiconductor film CsM₁(M₂)₃ on asubstrate comprising steps of:

-   -   providing a substrate;    -   depositing a high purity layer of M₁(M₂)₂;    -   depositing a high purity layer of CsM₂;    -   repeating the depositing steps until a desired number of        alternate layers is reached;    -   applying energy to activate a self-limiting chemical reaction of        M₁(M₂)₂ with CsM₂ forming the polycrystalline perovskite        semiconductor film CsM₁(M₂)₃;    -   wherein M₁ is selected from the group consisting of Sn, Pb and        combination thereof; and M₂ is selected from the group        consisting of I, Cl, Br and combinations thereof.

Yet, another embodiment of this invention is directed to the method forproducing a polycrystalline perovskite semiconductor film CsSnI₃ on asubstrate comprising steps of:

-   -   providing a substrate;    -   depositing a high purity layer of Sn(M₁)₂;    -   depositing a high purity layer of CsI;    -   repeat the depositing steps until a desired number of alternate        layers is reached;    -   applying a rapid thermal annealing to activate a self-limiting        chemical reaction of Sn(M₁)₂ with CsI forming the        polycrystalline perovskite semiconductor film CsSnI₃;    -   wherein M₁ is selected from the group consisting of I, Cl, Br        and combinations thereof.

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 shows the calculated electronic band structure of CsSnI₃. Six topvalence bands are displayed as thick curves while six bottom conductionbands are shown as thin curves. The theoretically calculated band-gapvalue from the first principle is known to be underestimated; theexperimentally measured band-gap value of ˜1.3 eV at 300 K was used.

FIG. 2 shows the high resolution Transmission Electron Microscope (TEM)image in a triple-domain area. Bottom-left inset: Electron diffractionpattern in a selected area where many domains are orientated indifferent directions. Top-left inset: an electron diffraction pattern ina selected area where a single large domain exists.

FIG. 3 shows the measured X-ray diffraction intensity (open circles) asa function of 2θ, compared with the calculated curve, matching mainfeatures of the γ-crystal structure of CsSnI₃ as indicated by “1, 2, and3”. The X-ray diffraction data (XRD) peaks from the ceramic substrateare removed for clarity.

FIG. 4 shows the photoluminescence (PL) spectra of CsI/SnI₂ layeredsamples cut from the same silicon substrate taken at different thermalannealing temperatures as indicated. The inset displays the integratedPL spectrum at different annealing temperatures.

FIG. 5 shows the measured absorption (thick curve) and photoluminescence(PL) spectra at room temperature (dash-curve) for a CsI/SnCl₂ layeredsample on a ceramic substrate.

DETAILED DESCRIPTION OF THE INVENTION

An effective and inexpensive method to synthesize large domain size highquality perovskite semiconductor thin films on large-area substratessuch as glass, ceramics and silicon are disclosed in the presentinvention.

Working Examples

The polycrystalline CsSnI₃ thin films were synthesized by followingsteps. The large domain sizes of 300 nm and larger (such as 3000 nm)were obtained. The domain size is defined as X, Y and Z, threedimensions. For example, domain sizes of 300 nm meant X, Y or Z is 300nm, respectively.

CsSnI₃ Thin Films Using SnI₂ and CsI

CsSnI₃ thin films using SnI₂ and CsI were synthesized by followingsteps:

providing a substrate;

depositing a high purity layer of SnI₂ using thermal evaporator,

depositing a high purity layer of CsI on the top of Sn_(I2) layer usinge-beam evaporator,

repeat the depositing steps until a desired number of alternate layerswas reached;

applying a rapid thermal annealing to activate a self-limiting chemicalreaction of CsI with SnI₂.

The depositing steps have been carried out in a vacuum with 10⁴˜10⁻⁶Torr, preferably 10⁻⁵ Torr; the deposition temperature ranged from 15°C. to 35° C., preferably 25° C.; the rapid thermal annealing timedranging from 10 s-20 s, preferably 15 s; the rapid thermal annealingtemperature ranged from 320° C. to 420° C., preferably 370° C.; therapid thermal annealing were carried out in an inert gas environment,such as, but not limited to N₂, Ar gas. A total layers ranged from 2 to100. The high purity is 99.99%.

While not wishing to be bound by theory, for the fixed 1-to-1stoichoimetric ratio, the chemical formula for the CsI/SnI₂ reaction is:CsI+SnI₂→CsSnI₃;

CsSnI₃ Thin Films Using SnCl₂ and CsI

CsSnI₃ thin films using SnCl₂ and CsI were synthesized by followingsteps:

providing a substrate;

depositing a high purity layer of SnCl₂ using thermal evaporator;

depositing a high purity layer of CsI on the top of SnCl₂ layer usinge-beam evaporator;

repeat the depositing steps until a desired number of alternate layerswas reached;

applying a rapid thermal annealing to activate a self-limiting chemicalreaction of CsI with SnCl₂.

The depositing steps have been carried out in a vacuum with 10⁻⁴˜10⁻⁶Torr, preferably 10⁻⁵ Torr; the deposition temperature ranged from 15°C. to 35° C., preferably 25° C.; the rapid thermal annealing timedranging from 10-20 s, preferably 15 s; the rapid thermal annealingtemperature ranged from 170° C. to 210° C., preferably 190° C.; therapid thermal annealing were carried out in an inert gas environment orin air, the inert gas were, but not limited to N₂, Ar gas; a totallayers ranged from 2 to 100.

While not wishing to be bound by theory, for the fixed 1-to-1stoichoimetric ratio, the chemical formula for the CsI/SnCl₂ reaction,three possible reactions that lead to CsSnI_(3-x)Cl_(x) (x=0, 1, 2, and3) structures are: 1) CsI+SnCl₂→CsSnICl₂, 2)2CsI+2SnCl₂→CsSnI₂Cl+CsSnCl₃, and 3) 3CsI+3 SnCl₂→CsSnI₃+2 CsSnCl₃.

CsSnI₃ Thin Films Using SnBr₂ and CsI

CsSnI₃ thin films using SnBr₂ and CsI were synthesized by followingsteps:

providing a substrate;

depositing a high purity layer of SnBr₂ using thermal evaporator;

depositing a high purity layer of CsI on the top of SnBr₂ layer usinge-beam evaporator;

repeat the depositing steps until a desired number of alternate layerswas reached;

applying a rapid thermal annealing to activate a self-limiting chemicalreaction of CsI with SnBr₂.

The depositing steps have been carried out in a vacuum with 10⁻⁴˜10⁻⁶Torr, preferably 10⁻⁵ Torr; the deposition temperature ranged from 15°C. to 35° C., preferably 25° C.; the rapid thermal annealing timedranging from 10-20 s, preferably 15 s; the rapid thermal annealingtemperature ranged from 170° C. to 210° C., preferably 190° C.; therapid thermal annealing were carried out in an inert gas environment orin air, the inert gas were, but not limited to N₂, Ar gas; a totallayers ranged from 2 to 100.

While not wishing to be bound by theory, for the fixed 1-to-1stoichoimetric ratio, the chemical formula for the CsI/SnBr₂ reaction,three possible reactions that lead to CsSnI_(3-x)Br_(x) (x=0, 1, 2, and3) structures are: 1) CsI+SnBr₂→CsSnIBr₂, 2)2CsI+2SnBr₂→CsSnI₂Br+CsSnBr₃, and 3) 3CsI+3 SnBr₂→CsSnI₃+2 CsSnBr₃.

Characterization of CsSnI₃ Thin Films

Different stoichoimetric ratios of CsI to SnI₂ or SnCl₂ have beenexperimented. The resulting films always gave a characteristic PLemission peak around 950 nm although its intensity varies slightly.

Various samples have been characterized using X-ray fluorescence (XRF)and energy dispersive X-ray analysis (EDS). For an example, with the1-to-1 ratio of CsI/SnI₂, an atom ratio was found by XRF to be 1:0.9:2.3for Cs:Sn:I after annealing, indicating slight loss of tin and iodineatoms during annealing. The EDS spectra were also acquired at variouslocations of annealed samples with no separated regions of CsI and SnI₂(or SnCl₂ in case of CsI/SnCl₂ layered samples).

Both CsI/SnI₂ and CsI/SnCl₂ layered thin film samples gave an intensecharacteristic PL around 950 nm. The possible side products for theCsI/SnCl₂ layered samples were CsSnCl₃, CsSnICl₂, and CsSnI₂Cl. Thefirst two had much larger band gaps than that of CsSnI₃, while the lastone had a smaller band-gap than that of CsSnI₃. Very weak PL relative tothe 950 nm emission were observed for a CsI/SnCl₂ layered sample andtheir PL peak positions were consistent with the calculated band-gapsfor CsSnCl₃ and CsSnICl₂. No PL or absorption features around 1.5 μmwere observed corresponding to the CsSnI₂Cl band gap. Hence thesepossible side products from CsI/SnCl₂ layered samples did not affect theintense band edge emission of CsSnI₃ reported here.

Optical absorption and photoluminescence (PL) methods were used todemonstrate that this compound is indeed a direct band-gapsemiconductor, consistent with the calculated from the first principles.The value of its band-gap was determined to be ˜1.3 eV at roomtemperature.

CASTEP simulation tool from Accelrys was used for this work. Thecomputational results on the total potential energy and electronicstates of a given crystal structure were based on the density functionalmodule CASTEP. Prior to the energy band structure calculation of acrystal structure, the type of crystal structure was determined by anenergy minimization procedure in which the potential energy wascalculated by varying a lattice scaling factor, by fine-tuning theSn—I—Sn (or Sn—Cl—Sn) titling angles in ab-plane and in c-direction, aswell as by changing Cs positions.

The band structure of CsSnI₃ based on the energy-minimized structuralcoordinates was shown in FIG. 1. Three important features of theelectronic states near the band edges should be pointed out.

First, it was clear shown that a direct band-gap semiconductor with aband-gap at Γ(x,y) symmetry point. Other symmetry points shown in FIG. 1were: S (−0.5a_(k), 0.5b_(k), 0), Z (0, 0, 0.5c_(k)), and R (−0.5a_(k),0.5b_(k), 0.5c_(k)), where a_(k)=π/a, b_(k)=π/b, and c_(k)=π/c.

Second, the curvature of the lowest conduction band (CB1) was about 2times smaller than the top valance band (VB1) indicating that theelectron effective mass is larger than that of holes.

Third, there was another conduction band (CB2) closely adjacent to CB1.They were parallel to each other in momentum space from Γ to S point.The electronic states of the CB1 was the p-orbital of the central tinatom of the SnI₆ octahedron; while the p- and s-orbital of the 6 outeriodine atoms of the octahedron equally contributed to the CB2 states.The electronic states of VB1 originated mainly from the p-orbital ofiodine atoms.

These thin films were characterized by the surface and cross-sectionscanning electron microscopy and transmission electron microscopy (TEM).The films were polycrystalline with a typical domain size of ˜300 nm.TEM images were taken from several selected areas, showing differentlattice spacing due to different crystal orientations.

FIG. 2 showed the TEM image of a CsSnI₃ film on a glass substrate in atriple-domain area. Electron diffraction patterns were also measured.

The polycrystalline films gave typical ring-like patterns, as shown bythe bottom-left inset in FIG. 2. In large domain areas, electrondiffraction patterns can present single crystal features. The measuredelectron diffraction patterns were displayed in the top-left inset ofFIG. 2.

CrystalMaker simulation package from CrystalMaker was used to generatethe electron diffraction pattern.

Twenty sets of crystal planes in the [201] direction of theoreticalcrystal structure CsSnI₃ were matched. The matching planes in sequenceof diffraction efficiency from 47 to 3% are: (−2 2 4), (−2 −2 4), (2, −24), (2 2 −4), (0 −4 0), (0 4 0), (0 −2 0), (0 2 0), (−1 −3 2), (1 −3−2), (−1 3 2), (1 3 −2), (−1 −1 2), (1 −1 −2), (−1 1 2), (1 1 −2), (−2 04), (−2 0 4), (−2 4 4) and (−2 4 4).

The crystal structure of polycrystalline films was further verified byX-ray diffraction (XRD) data and was shown in FIG. 3 as the open-circlecurve using a CsI/SnI₂ layered sample with a one-to-one stoichoimetricratio on a ceramic substrate. The numbers of “/1” and “2” indicated theexpected XRD features of the Sn—I—Sn bond tilting in the a-, andb-directions, respectively, while “3” represented the signature of theSn—I—Sn bond tilting in the c-direction. These features matched thecalculated XRD intensity (solid curve) for CsSnI₃.

PL spectra were extensively used to characterize the synthesized filmsunder various conditions. They were taken from a Nanolog system fromHoriba Jobin Yvon. The system consists of a light source (450 WXe-lamp), a double-grating excitation spectrometer to select a centralexcitation wavelength and its bandwidth, a sample compartment eitherfiber-coupled or in free-space to collect PL, and an emissionspectrometer to spectrally select desired emission to a photomultipliertube (Hamamatsu P2658P) coupled with single photon counting electroniccircuits. Photoexcitation level is low and is about 20 mW/cm².

Absorption spectra of CsSnI₃ thin films were measured by a Lambda-950UV-VIS-IR spectrometer equipped with a 60 mm integrating sphere fromPerkin Elmer.

The annealing temperature (T_(a)) and time duration (Δt_(a)) dependencesof the characteristic PL from CsSnI₃ thin films were studied. Theresults indicated that the peak position of PL did not depend on eitherannealing temperature or time duration used for the annealing. However,the intensity of PL was found strongly dictated by annealing conditions.

The optimal condition for the strongest PL intensity depended on a givensample. For a thin film with CsI/SnCl₂ layers on a glass substrate, atypical annealing temperature of ˜190° C. with time duration of ˜15 sresulted in a good polycrystalline film having very intense PL at ˜950nm. For a film with CsI/SnI₂ layers, the annealing temperature washigher than 190° C., since the melting temperature of SnI₂ (320° C.) ishigher than that of SnCl₂ (247° C.).

The PL spectra taken from the selected pieces of CsI/SnI₂ layeredsamples cut from a same Si substrate annealed at different temperaturesfor 20 s, were displayed in FIG. 4. The integrated area of PL spectrumat each annealing temperature was summarized in the inset of the figure,and it had two regions: below and above 330° C. which is close to themelting temperature of SnI₂.

The PL spectra indicated that the characteristic PL started as low as200° C. and reached to a local maximum at 260° C. before weakening tonear zero level. Right after 330° C., it immediately extended to thesecond maximum around 370° C., which was 4 times larger than the PLintensity at 260° C.

CsSnI₃ films would degrade in a few days under a normal ambientcondition without any protections. However, if they were stored in dryN₂ gas or encapsulated, the degradation was minimized.

Optical absorption spectra of CsSnI₃ thin films were measured at roomtemperature. A typical absorption spectrum of CsI/SnCl₂ layered filmsdeposited on ceramics was displayed in FIG. 5 along with the PL spectrumtaken from the same film. The absorption contribution from the substratewas removed and scatterings were considered through the integratingsphere.

The absorption spectrum reflected the nature of the inhomogeneity of thefilm in terms of composition and domain sizes. The value of theabsorption coefficient was zero before the PL emission peak, but steeplytook off after the PL emission peak. This behavior was a testimony forthe direct band-gap of CsSnI₃, as taught by J. I. Pankove “OpticalProcesses in Semiconductors”, Dover Publications, Inc., New York, 1971.

The shoulder riding on the absorption curve, ˜50 meV away from the PLpeak position, might associate with the second conduction band CB2although more work is needed to fully understand the nature ofabsorption in CsSnI₃ thin films.

It should be emphasized that the PL was very intense under a weakphotoexcitation indicating very high quantum efficiency, which issupportive to the direct band-gap assertion for the CsSnI₃ compound aspredicted by calculations from the first principles. The PL line shapewas inhomogeneously broadened with a spectral width of ˜50 meV.

In summary, a perovskite semiconductor CsSnI₃ thin film, another memberof the semiconductor family, has been deposited on a substrate. Themethods of depositing the high optical quality polycrystalline thinfilms were disclosed. Using the quantum mechanical simulation tools andthe methods of photoluminescence and optical absorption, it was verifiedthat the thin film was a direct band-gap semiconductor with a band-gapof ˜1.3 eV at 300 K°.

While the invention has been described in detail and with reference tospecific examples and the embodiments thereof, it will be apparent toone skilled in the art that various changes and modifications can bemade therein without departing from the spirit and scope thereof.

1. A polycrystalline perovskite semiconductor thin film comprisingCsM₁(M₂)₃; wherein M₁ is selected from the group consisting Sn, Pb andcombination thereof; and M₂ is selected from the group consisting of I,Cl, Br and combinations thereof.
 2. The polycrystalline perovskitesemiconductor thin film of claim 1, wherein M₁ is Cs; M₂ is Sn; M₃ is I;and M₁M₂(M₃)_(x) is CsSnI₃.
 3. The polycrystalline perovskitesemiconductor thin film of claim 1, wherein M₁ is Cs; M₂ is Sn; M₃ isCl; and M₁M₂(M₃)_(x) is CsSnCl₃.
 4. The polycrystalline perovskitesemiconductor thin film of claim 1, wherein M₁ is Cs; M₂ is Sn; M₃ isBr; and M₁M₂(M₃)_(x) is CsSnBr₃.
 5. The polycrystalline perovskitesemiconductor thin film of claim 2, wherein the polycrystallineperovskite semiconductor film having polycrystalline domains sizeranging from 300 nm to 3000 nm.
 6. The polycrystalline perovskitesemiconductor thin film of claim 2, wherein the polycrystallineperovskite semiconductor film having a band-gap ranging from 0 to 5 eV.7. A method of making a polycrystalline perovskite semiconductor thinfilm CsM₁(M₂)₃ on a substrate comprising steps of: providing asubstrate; depositing a high purity layer of M₁(M₂)₂; depositing a highpurity layer of CsM₂; repeating the depositing steps until a desirednumber of alternate layers is reached; applying energy to activate aself-limiting chemical reaction of M₁(M₂)₂ with CsM₂ forming thepolycrystalline perovskite semiconductor thin film CsM₁(M₂)₃; wherein M₁is selected from the group consisting of Sn, Pb and combination thereof;and M₂ is selected from the group consisting of I, Cl, Br andcombinations thereof.
 8. The method of claim 7, wherein the high puritylayer of M₁(M₂)₂ is deposited by using thermal evaporator; the highpurity layer of CsM₂ is deposited by using e-beam evaporator; and theenergy is selected from a thermal annealing, Ultra-violate, RFmicrowave, and laser; the depositing temperature ranges from 15° C. to35° C. and the depositing steps are carried out in vacuum ranging from10⁻⁴˜10⁻⁶ Torr.
 9. The method of claim 7, wherein the substrate isselected from glass, ceramic, and silicon.
 10. The method of claim 8,wherein the rapid thermal annealing temperature ranges from 170° C. to420° C., for a time ranging from 10 s-20 s.
 11. The method of claim 8,wherein the rapid thermal annealing is carried out in an inert gasenvironment, wherein the inert gas is selected from N₂, Ar orcombinations thereof.
 12. The method of claim 7, wherein the desiredalternate layers number ranges from 2 to
 100. 13. A method for producinga polycrystalline perovskite semiconductor thin film CsSnI₃ on asubstrate comprising steps of: providing a substrate; depositing a highpurity layer of Sn(M₁)₂; depositing a high purity layer of CsI; repeatthe depositing steps until a desired number of alternate layers isreached; applying a rapid thermal annealing to activate a self-limitingchemical reaction of Sn(M₁)₂ with CsI forming the polycrystallineperovskite semiconductor thin film CsSnI₃; wherein M₁ is selected fromthe group consisting of I, Cl, Br and combinations thereof.
 14. Themethod of claim 13, wherein the high purity layer of Sn(M₁)₂ isdeposited by using thermal evaporator; and the high purity layer of CsIis deposited by using e-beam evaporator.
 15. The method of claim 13,wherein the depositing steps are carried out in vacuum ranging from10⁻⁴˜10⁻⁶ Torr.
 16. The method of claim 13, wherein the depositingtemperature ranges from 15° C. to 35° C.
 17. The method of claim 13,wherein M₁ is I; the rapid thermal annealing is carried out in an inertgas environment; and wherein the inert gas is selected from N₂, Ar andcombinations thereof; and the rapid thermal annealing temperature rangesfrom 320° C. to 420° C., for a time ranging from 10 s-20 s.
 18. Themethod of claim 13, wherein M₁ is Cl; the rapid thermal annealing iscarried out in air or in an inert gas environment; and wherein the inertgas is selected from N₂, Ar and combinations thereof; and the rapidthermal annealing temperature ranges from 170° C. to 210° C., for a timeranging from 10 s-20 s.
 19. The method of claim 13, wherein the desiredalternate layers number ranges from 2 to
 100. 20. The method of claim13, wherein the substrate is selected from glass, ceramic, and silicon.